Semiconductor light-emitting element

ABSTRACT

A semiconductor light-emitting element is configured to emit ultraviolet light having a wavelength of 320 nm or shorter. Denoting a total area of a principal surface of a substrate as S0, an area on a p-type semiconductor layer in which a p-side contact electrode is formed as S1, an area on an n-type semiconductor layer in which an n-side contact electrode is formed as S2, a reflectivity of the p-side contact electrode for ultraviolet having a wavelength of 280 nm incident from a side of the p-type semiconductor layer as R1, and a reflectivity of the n-side contact electrode for ultraviolet light having a wavelength of 280 nm incident from a side of the n-type semiconductor layer as R2, (S1/S0)×R1+(S2/S0)×R2≥0.5, S1&gt;S2, and R1≤R2.

RELATED APPLICATION

Priority is claimed to Japanese Patent Application No. 2019-108417,filed on Jun. 11, 2019, and Japanese patent Application No. 2019-189327,filed on Oct. 16, 2019, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to semiconductor light-emitting elements.

2. Description of the Related Art

A light-emitting element for emitting deep ultraviolet light having awavelength of 355 nm or smaller includes an AlGaN-based n-type cladlayer, an active layer, and a p-type clad layer stacked on a substrate.An p-side electrode of Ni/Au is provided on the p-type clad layer forobtaining ohmic contact. The p-side electrode of Ni/Au has a lowreflectivity for deep ultraviolet light. Therefore, much of the deepultraviolet light traveling from the active layer toward the p-sideelectrode is absorbed by the p-side electrode, resulting in lower lightextraction efficiency. This is addressed by providing an opening on thep-type clad layer in which the p-side electrode is not formed andforming an Al reflection electrode having a high reflectivity for deepultraviolet light in the opening.

When an opening is formed for the reflection electrode, the contact areaof the p-side electrode is lowered, resulting in an increase in thecontact resistance of the p-side electrode. In a semiconductorlight-emitting element for emitting deep ultraviolet light, it ispreferred to realize an electrode structure having a low resistance anda high reflectivity.

SUMMARY OF THE INVENTION

The present invention addresses the above-described issue, and anillustrative purpose thereof is to improve the light extractionefficiency of a semiconductor light-emitting element.

A semiconductor light-emitting element according to an embodiment of thepresent invention includes: a n-type semiconductor layer of an n-typeAlGaN-based semiconductor material provided on a principal surface of asubstrate; an active layer of an AlGaN-based semiconductor materialprovided on the n-type semiconductor layer and configured to emitultraviolet light having a wavelength of 320 nm or shorter; a p-typesemiconductor layer provided on the active layer; a p-side contactelectrode provided on the p-type semiconductor layer; and an n-sidecontact electrode provided in a region on the n-type semiconductor layerdifferent from a region in which the active layer is formed. Denoting atotal area of the principal surface of the substrate as S0, an area onthe p-type semiconductor layer in which the p-side contact electrode isformed as S1, an area on the n-type semiconductor layer in which then-side contact electrode is formed as S2, a reflectivity of the p-sidecontact electrode for ultraviolet having a wavelength of 280 nm incidentfrom a side of the p-type semiconductor layer as R1, and a reflectivityof the n-side contact electrode for ultraviolet light having awavelength of 280 nm incident from a side of the n-type semiconductorlayer as R2, (S1/S0)×R1+(S2/S0)×R2≥0.5, S1>S2, and R1≤R2.

According to this embodiment, the area occupied by the active layer forimprovement of light emission efficiency can be enlarged so that thelight emission efficiency of the active layer can be improved, byconfiguring the area S1 in which the p-side contact electrode is formedto be larger than the area S2 in which the n-side contact electrode isformed. Meanwhile, it is not easy to increase the reflectivity R1 of thep-side contact electrode formed in a large area. The reflectivity R1 isconstrained to be equal to lower than the reflectivity R2 of the n-sidecontact electrode formed in a small area. Under these conditions, theembodiment realizes a reflectivity of 50% or higher relative to thetotal area S0 of the substrate both in the n-side contact electrode andin the p-side contact electrode, reduces the loss incurred by absorptionin the n-side or p-side contact electrode, and increases the lightextraction efficiency of the element as a whole.

The p-side contact electrode may include an Rh layer in contact with thep-type semiconductor layer.

The p-side contact electrode may include a transparent conductive oxidelayer in contact with the p-type semiconductor layer and a metal layerprovided on the transparent conductive oxide layer. Denoting atransmissivity of the transparent conductive oxide layer for ultraviolethaving a wavelength of 280 nm incident from a side of the p-typesemiconductor layer as T, a reflectivity of the metal layer forultraviolet having a wavelength of 280 nm incident from a side of thetransparent conductive oxide layer as R, the reflectivity R1 of thep-side contact electrode may be such that R1=RT².

The transparent conductive oxide layer may be an indium tin oxide layerhaving a thickness of 4 nm or smaller. The metal layer includes an Allayer having a thickness of 100 nm or larger.

The n-side contact electrode may include a Ti layer in contact with then-type semiconductor layer and having a thickness of not smaller than 1nm and not larger than 2 nm and an Al layer provided on the Ti layer andhaving a thickness of 100 nm or larger.

The semiconductor light-emitting element may be configured such that(S1+S2)/S0≥0.7, R1≥0.6, and R2≥0.8.

The p-type semiconductor layer may include a p-type contact layer incontact with the p-side contact electrode, and the p-type contact layermay be a p-type AlGaN or p-type GaN layer having an AlN ratio of 20% orlower, and a contact resistance between the p-type contact layer and thep-side contact electrode may be 1×10⁻²Ω·cm² or smaller.

A thickness of the p-type contact layer may be 20 nm or smaller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view schematically showing a configurationof a semiconductor light-emitting element according to an embodiment;

FIG. 2 is a top view schematically showing a configuration of thesemiconductor light-emitting element according to an embodiment;

FIG. 3 schematically shows the function of the reflection electrodes;

FIG. 4 schematically shows a step of manufacturing the semiconductorlight-emitting element;

FIG. 5 schematically shows a step of manufacturing the semiconductorlight-emitting element;

FIG. 6 schematically shows a step of manufacturing the semiconductorlight-emitting element;

FIG. 7 schematically shows a step of manufacturing the semiconductorlight-emitting element;

FIG. 8 schematically shows a step of manufacturing the semiconductorlight-emitting element;

FIG. 9 schematically shows a step of manufacturing the semiconductorlight-emitting element; and

FIG. 10 is a cross-sectional view schematically showing a configurationof a semiconductor light-emitting element according to anotherembodiment.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described by reference to the preferredembodiments. This does not intend to limit the scope of the presentinvention, but to exemplify the invention.

A detailed description will be given of embodiments of the presentinvention with reference to the drawings. The same numerals are used inthe description to denote the same elements and a duplicate descriptionis omitted as appropriate. To facilitate the understanding, the relativedimensions of the constituting elements in the drawings do notnecessarily mirror the relative dimensions in the actual light-emittingelement.

The embodiment relates to a semiconductor light-emitting elementconfigured to emit “deep ultraviolet light” having a central wavelengthλ of about 360 nm or shorter, which is a so-called deepultraviolet-light emitting diode (DUV-LED). To output deep ultravioletlight having such a wavelength, an aluminum gallium nitride(AlGaN)-based semiconductor material having a band gap of about 3.4 eVor larger is used. The embodiment particularly shows a case of emittingdeep ultraviolet light having a central wavelength λ of about 240 nm-320nm.

In this specification, the term “AlGaN-based semiconductor material”refers to a semiconductor material containing at least aluminum nitride(AlN) and gallium nitride (GaN) and shall encompass a semiconductormaterial containing other materials such as indium nitride (InN).Therefore, “AlGaN-based semiconductor materials” as recited in thisspecification can be represented by a compositionIn_(1−x−y)Al_(x)Ga_(y)N (0<x+y≤1, 0<x<1, 0<y<1). The AlGaN-basedsemiconductor material shall encompass AlGaN or InAlGaN. The“AlGaN-based semiconductor material” in this specification has a molarfraction of AlN and a molar fraction of GaN of 1% or higher, and,preferably, 5% or higher, 10% or higher, or 20% or higher.

Those materials that do not contain AlN may be distinguished byreferring to them as “GaN-based semiconductor materials”. “GaN-basedsemiconductor materials” mainly include GaN or InGaN. Similarly, thosematerials that do not contain GaN may be distinguished by referring tothem as “AlN-based semiconductor materials”. “AlN-based semiconductormaterials” include AlN or InAlN.

FIG. 1 is a cross sectional view schematically showing a configurationof a semiconductor light-emitting element 10 according to an embodiment.The semiconductor light-emitting element 10 includes a substrate 20, abase layer 22, an n-type semiconductor layer 24, an active layer 26, ap-type semiconductor layer 28, a p-side contact electrode 34, an n-sidecontact electrode 36, a protective layer 38, a p-side pad electrode 44,and an n-side pad electrode 46.

Referring to FIG. 1, the direction indicated by the arrow A may bereferred to as “vertical direction” or “thickness direction”. Further,the direction away from the substrate 20 may be defined as “upward”, andthe direction toward the substrate 20 may be defined as “downward”.

The substrate 20 is a substrate having translucency for the deepultraviolet light emitted by the semiconductor light-emitting element 10and is, for example, a sapphire (Al₂O₃) substrate. The substrate 20includes a first principal surface 20 a and a second principal surface20 b opposite to the first principal surface 20 a. The first principalsurface 20 a is a principal surface that is a crystal growth surface forgrowing the layers from the base layer 22 through the p-typesemiconductor layer 28. The second principal surface 20 b is a principalsurface that is a light extraction substrate for extracting the deepultraviolet light emitted by the active layer 26 outside. In avariation, the substrate 20 may be an AlN substrate or an AlGaNsubstrate.

The base layer 22 is provided on the first principal surface 20 a of thesubstrate 20. The base layer 22 is a foundation layer (template layer)to form the n-type semiconductor layer 24. For example, the base layer22 is an undoped AlN layer and is, specifically, an AlN (HT-AlN; HighTemperature-AlN) layer gown at a high temperature. The base layer 22 mayinclude an undoped AlGaN layer formed on the AlN layer. The base layer22 may be comprised only of an undoped AlGaN layer when the substrate 20is an AlN substrate or an AlGaN substrate. In other words, the baselayer 22 includes at least one of an undoped AlN layer and an undopedAlGaN layer.

The base layer 22 is formed such that the concentration of silicon (Si)as an n-type impurity is 5×10¹⁷ cm⁻³ or lower and is configured not tocontribute to conduction when electrons are injected from the n-sidecontact electrode 36 to the active layer 26. In other words, the baselayer 22 has a lower n-type impurity concentration than the n-typesemiconductor layer 24 and so has a lower conductivity (i.e., a higherresistivity).

The n-type semiconductor layer 24 is provided on the base layer 22. Then-type semiconductor layer 24 is an n-type AlGaN-based semiconductormaterial layer. For example, the n-type semiconductor layer 24 is anAlGaN layer doped with silicon (Si) as an n-type impurity. Thecomposition ratio of the n-type semiconductor layer 24 is selected totransmit the deep ultraviolet light emitted by the active layer 26. Forexample, the n-type semiconductor layer 24 is formed such that the molarfraction of AlN is 25% or higher, and, preferably, 40% or higher, or 50%or higher. The n-type semiconductor layer 24 has a band gap larger thanthe wavelength of the deep ultraviolet light emitted by the active layer26. For example, the n-type semiconductor layer 24 is formed to have aband gap of 4.3 eV or larger. It is preferable to form the n-typesemiconductor layer 24 such that the molar fraction of AlN is 80% orlower, i.e., the band gap is 5.5 eV or smaller. It is more preferable toform the n-type semiconductor layer 24 such that the molar fraction ofAlN is 70% or lower (i.e., the band gap is 5.2 eV or smaller). Then-type semiconductor layer 24 has a thickness of about 1 μm-3 μm. Forexample, the n-type semiconductor layer 24 has a thickness of about 2μm.

The n-type semiconductor layer 24 is formed such that the concentrationof Si as the impurity is not lower than 1×10¹⁸/cm³ and not higher than5×10¹⁹/cm³. It is preferred that to form the n-type semiconductor layer24 such that the Si concentration is not lower than 5×10¹⁸/cm³ and nothigher than 3×10¹⁹/cm³, and it is preferred to form it such that the Siconcentration is not lower than 7×10¹⁸/cm³ and not higher than2×10¹⁹/cm³. In one example, the Si concentration in the n-typesemiconductor layer 24 is around 1×10¹⁹/cm³ and is in a range not lowerthan 8×10¹⁸/cm³ and not higher than 1.5×10¹⁹/cm³.

The n-type semiconductor layer 24 includes a first top surface 24 a anda second top surface 24 b. The first top surface 24 a is where theactive layer 26 is formed, and the second top surface 24 b is where theactive layer 26 is formed. The first top surface 24 a and the second topsurface 24 b have different heights, and the height from the substrate20 to the first top surface 24 a is larger than the height from thesubstrate 20 to the second top surface 24 b. The region where the firsttop surface 24 a is located is defined as “first region W1”, and theregion where the second top surface 24 b is located is defined as“second region W2”. The second region W2 is adjacent to the first regionW1.

The active layer 26 is provided on the first top surface 24 a of then-type semiconductor layer 24. The active layer 26 is made of anAlGaN-based semiconductor material and forms a double heterojunctionstructure by being sandwiched by the n-type semiconductor layer 24 andthe p-type semiconductor layer 28. To output deep ultraviolet lighthaving a wavelength of 355 nm or shorter, the active layer 26 is formedto have a band gap of 3.4 eV or larger. For example, the AlN compositionratio of the active layer 26 is selected so as to output deepultraviolet light having a wavelength of 320 nm or shorter.

The active layer 26 may have, for example, a monolayer or multilayerquantum well structure. The active layer 26 is comprised of a stack of abarrier layer made of an undoped AlGaN-based semiconductor material anda well layer made of an undoped AlGaN-based semiconductor material. Theactive layer 26 includes, for example, a first barrier layer directly incontact with the n-type semiconductor layer 24 and a first well layerprovided on the first barrier layer. One or more pairs of the well layerand the barrier layer may be additionally provided between the firstbarrier layer and the first well layer. The barrier layer and the welllayer have a thickness of about 1 nm-20 nm, and has a thickness of, forexample, about 2 nm-10 nm.

The active layer 26 may further include an electron blocking layerdirectly in contact with the p-type semiconductor layer 28. The electronblocking layer is an undoped AlGaN-based semiconductor material layerand is formed such that, for example, the molar fraction of AlN is 40%or higher, and, preferably, 50% or higher. The electron blocking layermay be formed such that the molar fraction of AlN is 80% or higher ormay be made of an AlN-based semiconductor material that does notsubstantially contain GaN. The electron blocking layer has a thicknessof about 1 nm-10 nm. For example, the electron blocking layer has athickness of about 2 nm-5 nm.

The p-type semiconductor layer 28 is formed on the active layer 26. Thep-type semiconductor layer 28 is a p-type AlGaN-based semiconductormaterial layer or a p-type GaN-based semiconductor material layer. Forexample, the p-type semiconductor layer 28 is an AlGaN layer or a GaNlayer doped with magnesium (Mg) as a p-type impurity. The p-typesemiconductor layer 28 includes a p-type first clad layer 30, a p-typesecond clad layer 31, and a p-type contact layer 32. The p-type firstclad layer 30, the p-type second clad layer 31, and the p-type contactlayer 32 are formed such that their AlN ratios (proportions of AIN)differ each other.

The p-type first clad layer 30 is a p-type AlGaN layer having arelatively high AlN ratio, and the composition ratio thereof is selectedto transmit the deep ultraviolet light emitted by the active layer 26.The p-type first clad layer 30 is formed such that the molar fraction ofAlN is 40% or higher, and, preferably, 50% or higher, or 60% or higher.The AlN ratio of the p-type first clad layer 30 is, for example, similarto the AlN ratio of the n-type semiconductor layer 24 or larger than theAlN ratio of the n-type semiconductor layer 24. The AlN ratio of thep-type first clad layer 30 may be 70% or higher, or 80% or higher. Thep-type first clad layer 30 has a thickness of about 10 nm-100 nm and hasa thickness of, for example, about 15 nm-70 nm.

The p-type second clad layer 31 is a p-type AlGaN layer having a mediumAlN ratio and has an AlN ratio lower than that of the p-type first cladlayer 30 and higher than that of the p-type contact layer 32. The p-typesecond clad layer 31 is formed such that the molar fraction of AlN is25% or higher, and, preferably, 40% or higher, or 50% or higher. The AlNratio of the p-type second clad layer 31 is configured to be, forexample, about ±10% of the AlN ratio of the n-type semiconductor layer24. The second clad layer 31 has a thickness of about 5 nm-250 nm andhas a thickness of about 10 nm-150 nm. The p-type second clad layer 31may not be provided.

The p-type contact layer 32 is a p-type AlGaN layer or a p-type GaNlayer having a relatively low AlN ratio. The p-type contact layer 32 isformed such that the AlN ratio is 20% or lower in order to obtain properohmic contact with the p-side contact electrode 34. Preferably, thep-type contact layer 32 is formed such that the AlN ratio is 10% orlower, 5% or lower, or 0%. In other words, the p-type contact layer 32may be made of a p-type GaN-based semiconductor material that does notsubstantially contain AlN. As a result, the p-type contact layer 32could absorb the deep ultraviolet light emitted by the active layer 26.It is preferred to form the p-type contact layer 32 to be thin to reducethe quantity of absorption of the deep ultraviolet light emitted by theactive layer 26. The p-type contact layer 32 has a thickness of about 5nm-30 nm and has a thickness of about 10 nm-20 nm.

The p-side contact electrode 34 is provided on the p-type semiconductorlayer 28. The p-side contact electrode 34 can be in ohmic contact withthe p-type semiconductor layer 28 (i.e., the p-type contact layer 32)and is made of a material having a high reflectivity for the deepultraviolet light emitted by the active layer 26. The material havingsuch a property is limited. Our knowledge shows that rhodium (Rh) can beused. By configuring the p-side contact electrode 34 as an Rh layer, thecontact resistance relative to the p-type contact layer 32 can be 1×10⁻²Ω·cm² or smaller (e.g., 1×10⁻⁴ Ω·cm² or smaller), and the reflectivityof 60% or higher (e.g., about 60%-65%) for ultraviolet light having awavelength of 280 nm can be obtained. In this case, it is preferred thatthe thickness of the Rh layer forming the p-side contact electrode 34 be50 nm or larger or 100 nm or larger. In this specification, thereflectivity of the p-side contact electrode 34 for deep ultravioletlight having a wavelength of 280 nm incident from the side of the p-typesemiconductor layer 28 is also referred to as “first reflectivity R1”.

The p-side contact electrode 34 is formed inside the first region W1.The region in which the p-side contact electrode 34 is formed is definedas “third region W3”. The p-side contact electrode 34 is formed to be inohmic contact with the p-type semiconductor layer 28 over the entiretyof the third region W3 and to have a high reflectivity for deepultraviolet light over the entirety of the third region W3. It ispreferred that the p-side contact electrode 34 be formed to have auniform thickness over the entirety of the third region W3. This allowsthe p-side contact electrode 34 to function as a highly efficientreflection electrode that reflects the deep ultraviolet light from theactive layer 26 and guides it toward the second principal surface 20 bof the substrate 20 and to function as a low-resistance contactelectrode over the entirety of the third region W3.

The n-side contact electrode 36 is provided on the second top surface 24b of the n-type semiconductor layer 24. The n-side contact electrode 36is provided in the second region W2 different from the first region W1in which the active layer 26 is provided. The n-side contact electrode36 is made of a material that can be in ohmic contact with the n-typesemiconductor layer 24 and has a high reflectivity for the deepultraviolet light emitted by the active layer 26. The n-side contactelectrode 36 includes a first titanium (Ti) layer 36 a directly incontact with the n-type semiconductor layer 24, an aluminum (Al) layer36 b directly in contact with the first Ti layer 36 a, a second Ti layer36 c provided on the Al layer 36 b, and a titanium nitride (TiN) layer36 d provided on the second Ti layer 36 c.

The thickness of the first Ti layer 36 a is about 1 nm-10 nm and ispreferably 5 nm or smaller, and, more preferably, 1 nm-2 nm. Byconfiguring the first Ti layer 36 a to have a small thickness, theultraviolet reflectivity of the n-side contact electrode 32 as viewedfrom the n-type semiconductor layer 24 can be increased. The thicknessof the Al layer 36 b is about 100 nm-1000 nm and is preferably 200 nm orlarger. By configuring the Al layer to have a large thickness, theultraviolet reflectivity of the n-side contact electrode 36 can beincreased.

The second Ti layer 36 c and the TiN layer 36 d are provided to coverthe surface of the Al layer 36 b and prevent the Al layer 36 b frombeing oxidized when the n-side contact electrode 36 is annealed. Thetotal thickness of the second Ti layer 36 c and the TiN layer 36 d ispreferably 20 nm or larger, and, more preferably, 30 nm or larger. Byway of one example, the thickness of the second Ti layer 36 c is about10 nm-50 nm, and the thickness of the TiN layer 36 d is about 10 nm-50nm. The second Ti layer 36 c may not be provided, and only the TiN layer36 d may be provided.

The n-side contact electrode 36 is formed inside the second region W2.The region in which the n-side contact electrode 36 is formed is definedas “fourth region W4”. The n-side contact electrode 36 is formed to bein ohmic contact with the n-type semiconductor layer 24 over theentirety of the fourth region W4. By using a Ti/Al layer as the n-sidecontact electrode 36, contact resistance of 1×10⁻² Ω·cm² or smaller(e.g., 1×10⁻³ Ω·cm² or smaller) can be realized. The n-side contactelectrode 36 is formed to result in high reflectivity for deepultraviolet light over the entirety of the fourth region W4. Byconfiguring the first Ti layer 36 a to have a small thickness, thereflectivity of the n-side contact electrode 36 of 80% or higher (e.g.,about 85%-90%) for ultraviolet light having a wavelength of 280 nm canbe obtained. In this specification, the reflectivity of the n-sidecontact electrode 36 for ultraviolet light having a wavelength of 280 nmincident from the side of the n-type semiconductor layer 24 is alsoreferred to as “second reflectivity R2”.

It is preferred that the n-side contact electrode 36 be formed evenlyover the entirety of the fourth region W4. Stated otherwise, it ispreferred that the layers 36 a-36 d forming the n-side contact electrode36 be formed in a uniform thickness over the entirety of the fourthregion W4.

This allows the n-side contact electrode 36 to function as a highlyefficient reflection electrode that reflects the ultraviolet light fromthe active layer 26 and guides it toward the second principal surface 20b of the substrate 20 and to function as a low-resistance contactelectrode over the entirety of the fourth region W4. It is preferredthat the n-side contact electrode 36 does not contain gold (Au), whichcould cause lower the ultraviolet reflectivity.

The protective layer 38 is provided to cover the side surface (alsoreferred to as a mesa surface 12) of the active layer 26 and the p-typesemiconductor layer 28 and the surfaces of the p-side contact electrode34 and the n-side contact electrode 36. FIG. 1 shows that the mesasurface 12 of the active layer 26 and the p-type semiconductor layer 28is perpendicular to the substrate 20. Alternatively, the mesa surface 12may be sloped at a predetermined angle of slope with respect to thesubstrate 20. The angle of slope of the mesa surface 12 of the activelayer 26 and the p-type semiconductor layer 28 is, for example, not lessthan 40° and less than 55°.

The protective layer 38 is made of a dielectric material such as siliconoxide (SiO₂), silicon oxynitride (SiON), or aluminum oxide (Al₂O₃). Thethickness of the protective layer 38 is, for example, 100 nm or larger,200 nm or larger, 300 nm or larger, or 500 nm or larger. The thicknessof the protective layer 38 is, for example, 2 μm or smaller, 1 μm orsmaller, or 800 nm or smaller. By configuring the protective layer 38 tohave a large thickness, the surfaces of the respective layers formed onthe n-type semiconductor layer 24 are suitably covered and protected.

The protective layer 38 is made of a material having a lower refractiveindex for deep ultraviolet light than the active layer 26. Therefractive index of the AlGaN-based semiconductor material forming theactive layer 26 depends on the composition ratio and is about 2.1-2.56.Meanwhile, the refractive index of SiO₂ that could form the protectivelayer 38 for ultraviolet light having a wavelength of 280 nm is about1.4, the refractive index of SiON that could form the protective layer38 for ultraviolet light having a wavelength of 280 nm is about 1.4-2.1,and the refractive index of Al₂O₃ that could form the protective layer38 for ultraviolet light having a wavelength of 280 nm is about 1.8. Byproviding the protective layer 38 having a low refractive index, alarger portion of ultraviolet light can be totally reflected at theinterface between the active layer 26 and the protective layer 38 andguided toward the second principal surface 20 b of the substrate 20 thatis the light extraction surface. In the case of SiO₂, in particular, therefractive index difference from the active layer 26 is larger so thatthe reflection characteristic is further improved.

The p-side pad electrode 44 and the n-side pad electrode 46 (genericallyreferred to as pad electrodes) are portions bonded when thesemiconductor light-emitting element 10 is mounted on a packagesubstrate or the like. The p-side pad electrode 44 is provided on thep-side contact electrode 34 and is electrically connected to the p-sidecontact electrode 34 via a p-side opening 38 p that extends through theprotective layer 38. The n-side pad electrode 46 is provided on then-side contact electrode 36 and is electrically connected to the n-sidecontact electrode 36 via an n-side opening 38 n that extends through theprotective layer 38.

From the perspective of providing resistance to corrosion, the padelectrodes 44, 46 are configured to contain Au. For example, the padelectrodes 44, 46 are comprised of a Ni/Au, Ti/Au, or Ti/Pt/Au stackstructure. In the case the pad electrodes 44, 46 are bonded by gold-tin(AuSn), an AuSn layer embodying the metal joining member may be includedin the pad electrodes 44, 46.

FIG. 2 is a top view schematically showing a configuration of thesemiconductor light-emitting element 10 according to the embodiment.FIG. 1 described above corresponds to a B-B cross section of FIG. 2. Theouter shape of the semiconductor light-emitting element 10 is defined bythe outer circumference of the substrate 20 and is rectangular orsquare. An area S0 (also referred to as a total area) of a region Woccupied by the semiconductor light-emitting element 10 in a plan viewof FIG. 2 is equal to the area of the first principal surface 20 a orthe second principal surface 20 b of the substrate 20. The first regionW1 is a region in which the active layer 26 and the p-type semiconductorlayer 28 are formed. The area of the first region W1 is about 55%-65% ofthe total area S0. The second region W2 is a region in which the activelayer 26 or the p-type semiconductor layer 28 is not formed and is aregion excluding the first region W1. The area of the second region W2is about 35%-45% of the total area S0.

The third region W3 is a region in which the p-side contact electrode 34is formed and is a region slightly smaller than the first region W1. Thearea (also referred to as the first area S1) of the third region W3occupied by the p-side contact electrode 34 is about 45%-50% of thetotal area S0. The fourth region W4 occupied by the n-side contactelectrode 36 is a region in which the n-side contact electrode 36 isformed and is a region smaller than the second region W2. The area (alsoreferred to as the second area S2) of the fourth region W4 is about25%-30% of the total area S0. Therefore, the first area S1 (45%-50%)occupied by the p-side contact electrode 34 is larger than the secondarea S2 (25%-30%) occupied by the n-side contact electrode 36 (i.e.,S1>S2). For example, the first area S1 is configured to be 1.5 times thesecond area S2 or larger. Further, the area occupied by the p-sidecontact electrode 34 and the n-side contact electrode 36, i.e., a sum(S1+S2) of the first area S1 and the second area S2 is 70%-80% of thetotal area S0.

In the example shown in FIG. 2, the third region W3 in which the p-sidecontact electrode 34 is formed and the fourth region W4 in which then-side contact electrode 36 is formed are substantially rectangular. Thethird region W3 and the fourth region W4 need not necessarily be formedin a substantially rectangular shape but may have an arbitrary shape.For example, the third region W3 and the fourth region W4 may be formedin a comb-tooth shape such that the comb teeth of the regions areinserted into each another.

According to this embodiment, the reflection efficiency of the p-sidecontact electrode 34 and the n-side contact electrode 36 as reflectionelectrodes can be 50% or higher with respect to the total area S0 of thesemiconductor light-emitting element 10. In this specification, thereflection efficiency Rt of the element as a whole can be defined asRt=(S1/S0)×R1+(S2/S0)×R2. As already described, the total area of theprincipal surface of the substrate 20 is denoted as S0, the area (thefirst area) on the p-type semiconductor layer 28 in which the p-sidecontact electrode 34 is formed as S1, the area (the second area) on then-type semiconductor layer 24 in which the n-side contact electrode 36is formed as S2, the reflectivity (the first reflectivity) of the p-sidecontact electrode 34 for ultraviolet having a wavelength of 280 nmincident from the side of the p-type semiconductor layer 28 as R1, andthe reflectivity (the second reflectivity) of the n-side contactelectrode 36 for ultraviolet light having a wavelength of 280 nmincident from the side of the n-type semiconductor layer 24 as R2.

In one embodiment, the p-side contact electrode 34 is comprised of an Rhlayer having a thickness of 100 nm, and the n-side contact electrode 36is comprised of a Ti/Al/Ti/TiN layer having a thickness of 2 nm/600nm/25 nm/25 nm. In this embodiment, the first reflectivity R1 is about63%, and the second reflectivity R2 is about 89%. Therefore, the firstreflectivity R1 is equal to lower than the second reflectivity R2 (i.e.,R1≤R2). In this embodiment, the first area S1 of 45% or larger and thesecond area S2 of 25% or larger cause the reflection efficiency Rt ofthe element as a whole to be 50% or higher. For example, the first areaS1 of 47% and the second area S2 of 27% cause the reflection efficiencyRt of the element as a whole to be 54%.

FIG. 3 schematically shows the function of the reflection electrodes andillustrates ultraviolet light rays L1, L2, L3 emitted toward the secondprincipal surface 20 b of the substrate 20. The first light ray L1represents the light reflected by the p-side contact electrode 34 beforetraveling toward the second principal surface 20 b of the substrate 20.By configuring the first area S1 and the first reflectivity R1 of thep-side contact electrode 34 to be larger, the intensity of ultravioletlight like the first light ray L1 reflected by the p-side contactelectrode 34 and then being output outside can be increased. The secondlight ray L2 represents the light reflected by the n-side contactelectrode 36 before traveling toward the second principal surface 20 bof the substrate 20. By configuring the second area S2 and the secondreflectivity R2 of the n-side contact electrode 36 to be larger, theintensity of ultraviolet light like the second light ray L2 reflected bythe n-side contact electrode 36 and then being output outside can beincreased. The third light ray L3 represents the light reflected by themesa surface 12 of the active layer 26 or the p-type semiconductor layer28 before traveling toward the second principal surface 20 b of thesubstrate 20. By providing the protective layer 38 having a lowerrefractive index, the proportion of ultraviolet light totally reflectedby the mesa surface 12 can be increased, and the intensity ofultraviolet light like the third light ray L3 reflected by the mesasurface 12 and then being output outside can be increased.

Each of the light rays L1-L3 as illustrated shows a case in which thelight is reflected only once by the p-side contact electrode 34, then-side contact electrode 36, or the mesa surface 12. There is alsoultraviolet light that is reflected multiple times inside thesemiconductor light-emitting element 10 before being output outside.Further, there is also ultraviolet light reflected by both the p-sidecontact electrode 34 and the n-side contact electrode 36 before beingoutput outside. According to this embodiment, the intensity ofultraviolet light output from the second principal surface 20 b of thesubstrate 20 can be suitably increased by defining the reflectionefficiency Rt of the element as a whole and configuring thesemiconductor light-emitting element 10 such that the reflectionefficiency Rt is 50% or higher. In this way, the light extractionefficiency of the semiconductor light-emitting element 10 can beincreased according to this embodiment.

According to this embodiment, the area of the first region W1 in whichthe active layer 26 is provided can be enlarged by configuring the firstarea S1 occupied by the p-side contact electrode 34 to be larger thanthe second area S2 occupied by the n-side contact electrode 36. Byincreasing the proportion of the area of the first region W1 occupied bythe active layer 26, the light emission efficiency per a unit area ofthe substrate 20 can be increased, and the light extraction efficiencyof the semiconductor light-emitting element 10 can be increased.

According to this embodiment, the quantity of ultraviolet light absorbedby the p-type contact layer 32 can be reduced by reducing the thicknessof the p-type contact layer 32. In other words, the quantity ofabsorption of ultraviolet light reciprocally transmitted through thep-type contact layer 32 and reflected by the p-side contact electrode 34as indicated by the first light ray L1 can be reduced. In this way, thelight extraction efficiency of the semiconductor light-emitting element10 can be increased.

A description will now be given of a method of manufacturing thesemiconductor light-emitting element 10. FIGS. 4-9 schematically showsteps of manufacturing the semiconductor light-emitting element 10.Referring to FIG. 4, the base layer 22, the n-type semiconductor layer24, the active layer 26, the p-type semiconductor layer 28 (the p-typefirst clad layer 30, the p-type second clad layer 31, and the p-typecontact layer 32) are first formed on the first principal surface 20 aof the substrate 20 successively.

The substrate 20 is a sapphire (Al₂O₃) substrate and is a growthsubstrate for forming an AlGaN-based semiconductor material. Forexample, the base layer 22 is formed on the (0001) plane of the sapphiresubstrate. The base layer 22 includes, for example, an AlN (HT-AlN)layer grown at a high temperature and an undoped AlGaN (u-AlGaN) layer.The n-type semiconductor layer 24, the active layer 26, and the p-typesemiconductor layer 28 are layers made of an AlGaN-based semiconductormaterial, an AlN-based semiconductor material, or a GaN-basedsemiconductor material and can be formed by a well-known epitaxialgrowth method such as the metalorganic vapor phase epitaxy (MOVPE)method and the molecular beam epitaxial (MBE) method.

Subsequently, as shown in FIG. 5, a mask 50 is formed in the firstregion W1 on the p-type semiconductor layer 28, and the p-typesemiconductor layer 28 and the active layer 26 are dry-etched 60 fromabove the mask 50. The mask 50 can be formed by using, for example, awell-known photolithographic technique. The dry-etching 60 is performeduntil the n-type semiconductor layer 24 is exposed in the second regionW2. In this way, the second top surface 24 b of the n-type semiconductorlayer 24 is formed. The active layer 26 and the p-type semiconductorlayer 28 having the mesa surface 12 are formed in the first region W1.For example, reactive ion etching using an etching gas turned into aplasma can be used in the step of forming the mesa surface 12. Forexample, inductively coupled plasma (ICP) etching may be used. The mask50 is removed after the dry-etching 60 is performed.

Subsequently, as shown in FIG. 6, a mask 52 having an opening 51 in thethird region W3 is formed, and the p-side contact electrode 34 is formedin the third region W3 on the p-type semiconductor layer 28. The mask 52can be formed by using, for example, a well-known photolithographytechnique. The p-side contact electrode 34 can be formed by sputteringor electron beam (EB) deposition. Proper ohmic contact can beestablished between the p-type semiconductor layer 28 and the p-sidecontact electrode 34 by forming the p-side contact electrode 34 on thep-type semiconductor layer 28 immediately after the mesa surface 12 isformed. The p-side contact electrode 34 is annealed after the p-sidecontact electrode 34 is formed and the mask 52 is removed.

Subsequently, as shown in FIG. 7, a mask 54 having an opening 53 in thefourth region W4 is formed, and the n-side contact electrode 36 isformed in the fourth region W4 on the second top surface 24 b of then-type semiconductor layer 24. The mask 54 can be formed by using, forexample, a well-known photolithography technique. First, the first Tilayer 36 a, the Al layer 36 b, and the second Ti layer 36 c are formedon the second top surface 24 b of the n-type semiconductor layer 24.These layers are formed by sputtering or EB deposition. By forming theselayers by sputtering, metal layers having a smaller film density can beformed than by using EB deposition, and excellent contact resistance isrealized at a relatively low annealing temperature.

The mask 54 is then removed. Nitrogen (N) atoms are supplied to thesurface of the second Ti layer 36 c to nitride the surface of the secondTi layer 36 c, by processing the surface of the second Ti layer 36 cwith an ammonia (NH₃) gas plasma. In this way, the TiN layer 36 d isformed. The temperature of the plasma process for forming the TiN layer36 d is preferably below the melting point of Al (about 660° C.), and,more preferably, below 300° C., for example.

The n-side contact electrode 36 is then annealed. The n-side contactelectrode 36 is preferably annealed at a temperature below the meltingpoint of Al (about 660° C.) and is preferably annealed at a temperaturenot lower than 560° C. and not higher than 650° C. The film density ofthe Al layer of less than 2.7 g/cm³ and the annealing temperature of notlower than 560° C. and not higher than 650° C. ensure that the contactresistance of the n-side contact electrode 36 is 1×10⁻² Ω·cm² orsmaller. The annealing temperature of not lower than 560° C. and nothigher than 650° C. enhances the flatness of the n-side contactelectrode 36 after annealing and provides an ultraviolet reflectivity of80% or higher.

Subsequently, the protective layer 38 is formed as shown in FIG. 8. Theprotective layer 38 is formed to cover the entirety of the top surfaceof the element structure. The protective layer 38 is formed to cover thesurface of the p-side contact electrode 34 and the n-side contactelectrode 36 and to cover the exposed surfaces of the active layer 26and the p-type semiconductor layer 28 including the mesa surface 12. Theprotective layer 38 is provided to cover at least a part of the secondtop surface 24 b of the n-type semiconductor layer 24.

Subsequently, as shown in FIG. 9, a mask 56 having openings 55 p, 55 nis formed, and the p-side opening 38 p and the n-side opening 38 n areformed by removing portions of the protective layer 38. The mask 56 canbe formed by using, for example, a well-known photolithographytechnique. The openings 55 p, 55 n of the mask 56 are positioned abovethe p-side contact electrode 34 and the n-side contact electrode 36,respectively. Portions of the protective layer 38 can be removed bydry-etching 62 using a CF-based etching gas such as a hexafluoroethane(C₂F₆) gas. In this dry-etching step, the Rh layer of the p-side contactelectrode 34 and the TiN layer 36 d of the n-side contact electrode 36function as stop layers for the dry-etching 62. This prevents a damageto the p-side contact electrode 34 and the n-side contact electrode 36and maintains the low-resistance and high-reflectivity contactelectrodes.

Subsequently, the p-side pad electrode 44 is formed in the p-sideopening 38 p on the p-side contact electrode 34, and the n-side padelectrode 46 is formed in the n-side opening 38 n on the n-side contactelectrode 36. For example, the pad electrodes 44, 46 can be formed byfirst depositing the Ni layer or the Ti layer and then depositing the Aulayer thereon. A further metal layer may be provided on the Au layer.For example, an Sn layer, an AuSn layer, or a stack structure of Sn/Aulayers may be formed. The pad electrodes 44, 46 may be formed by usingthe mask 56 or formed by using further resist mask different from themask 56. After the pad electrodes 44, 46 are formed, the mask 56 or thefurther resist mask is removed.

Through the steps described above, the semiconductor light-emittingelement 10 shown in FIG. 1 is completed. According to this embodiment,the p-side contact electrode 34 comprised of the Rh layer can be placedin suitable ohmic contact with the p-type contact layer 32, and contactresistance of 1×10⁻² Ω·cm² or smaller can be realized, by forming thep-side contact electrode 34 before forming and annealing the n-sidecontact electrode 36.

FIG. 10 is a cross-sectional view schematically showing a configurationof a semiconductor light-emitting element 110 according to anotherembodiment. This embodiment differs from the embodiment described abovein that a p-side contact electrode 134 is configured as a three-layerstructure. A description will now be given of the semiconductorlight-emitting element 110, highlighting the difference from theembodiment described above.

The semiconductor light-emitting element 110 includes the substrate 20,the base layer 22, the n-type semiconductor layer 24, the active layer26, the p-type semiconductor layer 28, a p-side contact electrode 134,the n-side contact electrode 36, the protective layer 38, the p-side padelectrode 44, and the n-side pad electrode 46.

The p-side contact electrode 134 includes a transparent conductive oxide(TCO) layer 134 a and metal layers 134 b, 134 c provided on the TCOlayer 134 a. The term “transparent conductive oxide” refers to amaterial generally called a transparent conductive oxide and refers to amaterial that is transparent (highly transmissive) to visible light.Therefore, a “transparent conductive oxide” is not necessarilytransparent to the deep ultraviolet light emitted by the active layer26. For example, the transparent conductive oxide can absorb ultravioletlight having a wavelength of 280 nm to a certain degree. It is preferredthat the TCO layer 134 a be made of tin oxide (SnO₂), zinc oxide (ZnO),indium tin oxide (ITO) or the like. The TCO layer 134 a is preferablyhighly conductive ITO layer. It is preferred to form the TCO layer 134 ato be thin to reduce the quantity of absorption of the deep ultravioletlight emitted by the active layer 26. It is preferred that the TCO layer134 a be, for example, an ITO layer having a thickness of 4 nm orsmaller and be an ITO layer having a thickness of about 3 nm.

The metal layers 134 b, 134 c are formed to have a high reflectivity forultraviolet light. The first metal layer 134 b is a palladium (Pd) layeror an Ni layer, and the second metal layer 134 c is an Al layer. Thethickness of the second metal layer 134 c is about 100 nm-1000 nm and ispreferably 200 nm or larger. By configuring the Al layer to have a largethickness, the ultraviolet reflectivity of the p-side contact electrode134 can be increased. The first metal layer 134 b is provided toincrease the adhesiveness between the TCO layer 134 a and the secondmetal layer 134 c. It is preferred to form the first metal layer 134 bto be thin to reduce the quantity of absorption of ultraviolet light.The first metal layer 134 b is formed to have a thickness of 5 nm orsmaller, 3 nm or smaller, or 2 nm or smaller.

According to this embodiment, the contact resistance of the p-sidecontact electrode 134 can be 1×10⁻² Ω·cm² or smaller by using the TCOlayer 134 a placed in ohmic contact with the p-type contact layer 32.Further, by combining the Pd layer having a small thickness and the Allayer having a large thickness, the adhesiveness of the Al layer withthe TCO layer 134 a is increased, and the reflectivity of the metallayers 134 b, 134 c for the ultraviolet light having a wavelength of 280nm can be 90% or higher.

In this embodiment, the TCO layer 134 a is inserted between the p-typesemiconductor layer 28 and the metal layers 134 b, 134 c. Therefore,loss is incurred as a result of the light being reciprocally transmittedthrough the TCO layer 134 a. It is therefore necessary to consider theloss incurred as a result of the light being reciprocally transmittedthrough the TCO layer 134 a to determine the first reflectivity R1 ofthe p-side contact electrode 134 for the ultraviolet light incident fromthe side of the p-type semiconductor layer 28. Denoting the reflectivityof the metal layers 134 b, 134 c as R, and the one-way transmissivity ofthe TCO layer 134 a as T, the first reflectivity R1 of the p-sidecontact electrode 134 as a whole will be such that R1=RT². When the TCOlayer 134 a is made of ITO, for example, the transmissivity of an ITOlayer having a thickness of 3 nm for ultraviolet light having awavelength of 280 nm is about 87%, and the transmissivity of an ITOlayer having a thickness of 4 nm for ultraviolet light having awavelength of 280 nm is about 83%. Given that the reflectivity of themetal layers 134 b, 134 c for ultraviolet light having a wavelength of280 nm is 90%, the first reflectivity R1 of the p-side contact electrode134 occurring when the ITO layer having a thickness of 4 nm is used willbe about 68%, and the first reflectivity R1 of the p-side contactelectrode 134 occurring when the ITO layer having a thickness of 4 nm isused will be about 62%.

Therefore, it is possible to ensure that (S1+S2)/S0≥0.7, R1≥0.6, andR2≥0.8 in this embodiment as similar in the foregoing embodiment. Thisensures that the conditions requiring (S1/S0)×R1+(S2/S0)×R2≥0.5, S1>S2,and R1≤R2 are met, and the reflection efficiency Rt of the element as awhole is 50% or higher. As a result, the semiconductor light-emittingelement 110 having an excellent light extraction efficiency can beprovided.

In one embodiment, the p-side contact electrode 34 is comprised of anITO/Pd/Al layer having a thickness of 3 nm/2 nm/200 nm, and the n-sidecontact electrode 36 is comprised of a Ti/Al/Ti/TiN layer having athickness of 2 nm/600 nm/25 nm/25 nm. In this embodiment, the firstreflectivity R1 is 68%, and the second reflectivity R2 is 89%. Byconfiguring the first area S1 to be 45% or larger and the second area S2to be 25% or larger in this embodiment, the reflection efficiency Rt ofthe element as a whole will be 50% or higher. Given that, for example,the first area S1 is 47% and the second area S2 is 27%, the reflectionefficiency Rt of the element as a whole will be 56%.

The semiconductor light-emitting element 110 can be manufactured bysteps similar to those of the semiconductor light-emitting element 10according to the embodiment described above. The TCO layer 134 a, thefirst metal layer 134 b, and the second metal layer 134 c forming thep-side contact electrode 134 can be formed by sputtering or electronbeam (EB) deposition. In this embodiment, the p-side contact electrode134 may be formed before forming the n-side contact electrode 36, or thep-side contact electrode 134 may be formed after forming the n-sidecontact electrode 36.

Described above is an explanation based on an exemplary embodiment. Theembodiment is intended to be illustrative only and it will be understoodby those skilled in the art that various design changes are possible andvarious modifications are possible and that such modifications are alsowithin the scope of the present invention.

What is claimed is:
 1. A semiconductor light-emitting elementcomprising: a n-type semiconductor layer made of an n-type AlGaN-basedsemiconductor material provided on a principal surface of a substrate;an active layer made of an AlGaN-based semiconductor material providedon the n-type semiconductor layer and configured to emit ultravioletlight having a wavelength of 320 nm or shorter; a p-type semiconductorlayer provided on the active layer; a p-side contact electrode providedon the p-type semiconductor layer; and an n-side contact electrodeprovided in a region on the n-type semiconductor layer different from aregion in which the active layer is formed, wherein denoting a totalarea of the principal surface of the substrate as S0, an area on thep-type semiconductor layer in which the p-side contact electrode isformed as S1, an area on the n-type semiconductor layer in which then-side contact electrode is formed as S2, a reflectivity of the p-sidecontact electrode for ultraviolet having a wavelength of 280 nm incidentfrom a side of the p-type semiconductor layer as R1, and a reflectivityof the n-side contact electrode for ultraviolet light having awavelength of 280 nm incident from a side of the n-type semiconductorlayer as R2, (S1/S0)×R1+(S2/S0)×R2≥0.5, S1>S2, and R1≤R2.
 2. Thesemiconductor light-emitting element according to claim 1, wherein thep-side contact electrode includes an Rh layer in contact with the p-typesemiconductor layer.
 3. The semiconductor light-emitting elementaccording to claim 1, wherein the p-side contact electrode includes atransparent conductive oxide layer in contact with the p-typesemiconductor layer and a metal layer provided on the transparentconductive oxide layer, and denoting a transmissivity of the transparentconductive oxide layer for ultraviolet having a wavelength of 280 nmincident from a side of the p-type semiconductor layer as T, areflectivity of the metal layer for ultraviolet having a wavelength of280 nm incident from a side of the transparent conductive oxide layer asR, the reflectivity R1 of the p-side contact electrode is such thatR1=RT².
 4. The semiconductor light-emitting element according to claim3, wherein the transparent conductive oxide layer is an indium tin oxidelayer having a thickness of 4 nm or smaller, and the metal layerincludes an Al layer having a thickness of 100 nm or larger.
 5. Thesemiconductor light-emitting element according to claim 1, wherein then-side contact electrode includes a Ti layer in contact with the n-typesemiconductor layer and having a thickness of not smaller than 1 nm andnot larger than 2 nm and an Al layer provided on the Ti layer and havinga thickness of 100 nm or larger.
 6. The semiconductor light-emittingelement according to claim 1, wherein (S1+S2)/S0≥0.7, R1≥0.6, andR2≥0.8.
 7. The semiconductor light-emitting element according to claim1, wherein the p-type semiconductor layer includes a p-type contactlayer in contact with the p-side contact electrode, and the p-typecontact layer is a p-type AlGaN or p-type GaN layer having an AlN ratioof 20% or lower, and a contact resistance between the p-type contactlayer and the p-side contact electrode is 1×10⁻² Ω·cm² or smaller. 8.The semiconductor light-emitting element according to claim 7, wherein athickness of the p-type contact layer is 20 nm or smaller.